The present invention relates to an exposure mask for forming a three-dimensional shape such as an optical lens array by means of exposure, and a fabrication method for the exposure mask.
As one of fabrication methods for micro optical parts such as microlens arrays for use in applied products of imaging devices such as CCDs (Charge Coupled Devices) and LCDs (Liquid Crystal Displays), there is a method which uses photolithography techniques employed in the fabrication of semiconductors and liquid crystal devices.
Namely, this method three-dimensionally processes a photoresist by giving the desired exposure amount distribution to the photoresist which is a photosensitive material, and three-dimensionally processes a silicon or glass substrate or the like by etching using the photoresist as a mask.
A first example of a photomask used in this lithography process is realized by multiple exposure using a plurality of masks as shown in FIG. 29. An exposure method using this technique will be described on a one-dimensional basis with reference to FIG. 29. A final exposure distribution is assumed to be D(X) in FIG. 29.
First, an exposure amount E[1] is given to a region <1> through a mask (1) in FIG. 29. Then, an exposure amount E[2] is given to a region <2> through a mask (2). At this time, a total exposure D1 of the region <1> becomes E[1]+E[2]. Further, a mask (3), a mask (4), . . . , mask (n), which are not shown, are respectively sequentially exposed with exposure amounts E[3], E[4], . . . , E[n], whereby a final exposure amount D[i] of a region i becomes D[i]=E[i]+E[i+1]+ . . . +E[n] and the desired discrete exposure amount distribution is obtained. In this case, the number of masks “n” corresponds to the position resolution of the exposure amount, and for example, if n=10, 10 gray scale exposure amount steps are obtained.
As a second example other than the multiple exposure using a plurality of masks, a method has recently been developed which obtains the desired exposure amount distribution through one exposure by using a so-called transmissive type of gray-tone mask which is a mask called High Energy Beam Sensitive (HEBS) glass having a mask substrate whose transmittance is given a continuous distribution as described in Japanese Patent Application Publication No. 2002-189280 and the specification of U.S. Pat. No. 4,567,104. A conceptual diagram is shown in FIG. 30.
As a third example, the specifications of U.S. Pat. No. 3,373,518, U.S. Pat. No. 5,310,623 and U.S. Pat. No. 6,335,151, which will be mentioned later, have proposed the use of a mask which is formed of binary patterns so that its pattern sizes are controlled to control light intensity on an exposure surface.
From among the above-mentioned methods, because the technique of the first example using multiple exposure with a plurality of masks is a plurality of multiple exposures and temporally needs multiple-step exposure, a staircase-like shape remains in any obtainable cumulative exposure amount distribution. In addition, the obtainable number of exposure gray scales is the number of mask, i.e., the number of times of exposure, and actually corresponds to approximately 10 steps which result in the problem that a sufficient number of gray scales are not obtainable. In addition, mask cost proportional to the complexity of the exposure process and the number of masks occurs, resulting in various problems.
The one-exposure method of the second example which uses the gray-tone mask is capable of providing an approximately continuous exposure amount distribution, but in general, this gray-tone mask is extremely difficult to fabricate and needs a special substrate material and special deposition process techniques. This results in extremely high mask cost. In addition, it has been pointed out that the special film material tends to suffer variations with time due to heat and has the problem of performance stability during use (thermal stability due to exposure light absorption).
The one-exposure method which uses the mask of the third example does not use a special semi-transparent light-blocking film and is made of so-called ordinary binary patterns, but the light intensity on the exposure surface is set to vary approximately continuously with respect to positions. In the specifications of U.S. Pat. No. 3,373,518 and U.S. Pat. No. 5,310,623, the mask is separated into sub-pixels which are divided vertically and horizontally with respect to the direction of the optical axis, and each sub-pixel is divided into color tone elements which are based on gray scale resolution, and light intensity is controlled by means of the ratio of transmissive ones to non-transmissive ones of these color tone elements.
Therefore, in the specifications of U.S. Pat. No. 3,373,518 and U.S. Pat. No. 5,310,623, since the above-mentioned color tone element is 0.2 μm on a side, the sub-pixel which is the unit of light intensity modulation is 2 μm long on a side. This leads to the problem that a sufficient number of intensity modulations cannot be obtained with respect to the unit lens size (˜10 μm) of a microlens array of, for example, a liquid crystal projector, and it is impossible to deal with the formation of microlenses which are becoming smaller and smaller.
It is appropriate to apply a reduction projection exposure method in order to form far finer three-dimensional structures. In this case, however, instead of design which takes into account only the opening area in each sub-pixel, the sub-pixel size (sub-pixel pitch) must be set to not greater than a pitch defined optically, so that an image of the opening pattern in the sub-pixel is not formed. The specifications of U.S. Pat. No. 3,373,518 and U.S. Pat. No. No. 5,310,623 mainly assume proximity exposure as a premise, and do not make specific reference to any projection exposure method.
In the specification of U.S. Pat. No. 6,335,151, the numerical analysis of reduction projection exposure lithography is shown, and the opening centers of individual sub-pixels are concentrically arranged. For this reason, pitches in the X direction, the Y direction and oblique directions irregularly vary below a resolution limit and ripple-like light intensity occurs at locations where different pitches appear, so that the surface of a formed three-dimensional shape becomes rugged and greatly affects the performance of optical lenses. In addition, in this concentric arrangement, if pattern arrangement is performed so that a square lens array which optically uses its four corners as well can be formed, patterns are extremely difficult to arrange at the four corners.
Furthermore, in these specifications of U.S. Pat. No. 3,373,518, U.S. Pat. No. 5,310,623 and U.S. Pat. No. 6,335,151, as to pattern writing using EB (electron beam) in the fabrication of a mask for color tone element unit patterns in the sub-pixels, pattern design based on spot beam scanning (vector scanning or raster scanning) is performed, resulting in design digitized in the units of color tone elements. Accordingly, the openings in the sub-pixels become polygonal and in an actually fabricated mask, diffraction and scattering phenomena at mask pattern edges cannot be ignored. This leads to the problem that the mask transmittance cannot be represented by a simple pattern density and the desired mask transmittance cannot be achieved.
In addition, in the specification of U.S. Pat. No. 6,335,151, a resist is exposed and developed in advance by the use of a mask having no patterns, and pattern design based on the correlation between exposure amount and photoresist film thickness after development is performed. However, exposure with an actual gray-tone mask and exposure without patterns differ in flare light intensity occurring on the exposure surface. Accordingly, if the mask designed in the procedures of the specification of U.S. Pat. No. 6,335,151 is employed, exposure with the mask receives the influence of fog exposure due to unexpected flare. This results in the problem that the controllability of resist height is inferior at a location where mask transmittance is low.